The invention relates to granulation,. i.e. transforming particles into granules each comprising several particles.
Granulation of fine powder materials is applied within various industries. Granulation is used to transform a powder with poor free flowing capability into a product with proper free flowing capabilities. Another reason for granulation is to avoid that the final product has an unwanted dust emission.
In the pharmaceutical industry granulation is used mainly for treating materials to be pressed into tablets.
In the food industries, incl. the dairy industry, granulation is used, inter alia to improve dispersibility and reconstitutional properties.
In the chemical industry granulation is used to improve the handling characteristics of the materials and the exactness of the dosage. Furthermore, granulation is used to bind active dust materials to harmless base substances.
Many chemical products, incl. detergents, fertilizers, catalysts, pesticides and dyestuffs, are wanted in a granulated form.
In the present specification, the term granulation is used including what is often termed agglomeration.
In general, all granulation processes follow the same basic technique. The material to be granulated is wetted with a solvent or a solution to make the primary particles forming the material tacky A binder may be present in the particles or in the solution, or the ability of becoming tacky when wet may be inherent in the particles. The wet and tacky particles adhere to each other when brought into mutual contact and the junction is fixed by drying the material. Various materials differ in their ability to absorb the solvent or solution without getting totally wet and they become adhesive at different product humidity levels, so obviously not all materials are equally easy to granulate. To make the process more robust and easier to control, normally a combination of materials is used with different abilities to become tacky. Another approach to obtain adhesion between the particles is to use a mechanical force simultaneously with the wetting.
Granulation can be performed, using methods of different types.
One of these types is mechanical granulation. In mechanical granulation the solvent addition is low and the force, applied on the product, high. Mechanical granulations are classified in two main groups, high shear and low shear granulation. The advantage with high shear granulation is, that a broad range of formulations can be granulated. The granules produced in a high shear granulation are hard and compact, and the process is fast. One of the shortcomings of high shear granulation is the broad particle size distribution, which normally demands sizing by milling. The large sized compact granules (lumps) often found also give problems in the subsequent drying process.
Low shear granulation (such as ROTO-PROCESSOR(trademark) from Aeromatic-Fielder) gives less compact granules and a more narrow particle size distribution. This type of granulation does not demand milling and the granules produced by it are easy to dry.
A disadvantage with the low shear granulation is the difficulty in scaling up and the limit in the size of the batch.
In both, high shear and low shear granulation, pure solvent or solvent combined with binder can be used.
Another type of conventional granulation is fluid bed granulation. In fluid bed granulation, the product is kept fluidized while the solvent or solution is applied by means of a nozzle above or within the fluidized layer. The process of getting the single particles of the product together has to rely on the random movement of the product and on the fact that the product particles are wet enough to adhere to each other. The fact that various parts of the product absorb the solvents at a different rate and get tacky at different levels of product humidity, prevents certain formulations, to be granulated in a fluid bed. An advantage with fluid bed granulation is that the particle distribution can be more narrow than in mechanical granulation and the process is easy to scale up. Critical parameters for fluid bed granulation are the product layer thickness and the distribution of the solution spray over the product.
A disadvantage with fluid bed granulation can be that the granules are not so compact and stable as the granule produced in the mechanical process. Another disadvantage is, that when treating products having a critical wetting rate, the process is difficult to control. During the granulation process the particle size is changing, which means that the volume flow has to be increased at the same rate as the particle size growth to ensure the same grade of suspension of the product layer throughout the process. Because the granule produced in fluid bed granulation is not as stable as in the mechanical granulation process, the granule is vulnerable to attrition during the drying process. Examples of fluid bed granulation are described in U.S. Pat. No. 5,695,701 (Niro).
Several equipments have been designed to employ a combination of mechanical granulation with fluid bed granulation. The fundamental feature of this hybrid granulation process is to add force when the primary particles are brought into contact. The advantage with this granulation is, that the amount of solvent, added to the product, is less than in a pure fluid bed granulation for which reason the drying time of the granules is shorter. However, the control of this hybrid granulation is complicated. A problem with this granulation process is also the difficulty to scale up the batch size, which is rather limited, as in the mechanical granulation process.
Granulation is also performed as an integrated part of a spray drying process in which fine particles produced by spray drying are recycled to the spraying zone in the spray drying apparatus for contacting droplets and wet particles and subsequent drying. However, said method involves a risk of heat-damaging and is, moreover, only suitable when at least a substantial part of the starting material is present as a liquid. Besides, plant investments are high and only acceptable when a spray drying process is necessitated to transform a starting liquid into dry solid. Thus. such processes are out of .consideration when the problem is to granulate a material already present as particles
Although the process of the present invention does not involve treatment of particulates while these are in a fluidized bed, the above described fluidized bed granulation processes may be regarded as the closest prior art relative to the present invention.
Various equipment has been used or suggested for fluidized bed granulation. Thus, in published German Patent Application DE 3323418 A1 an apparatus is disclosed for fluidized bed processes primarily for coating of bodies such as tablets, however, also granulation is mentioned as a hypothetical possibility.
A feature of this fluidized bed apparatus is a vertical pipe arranged over an upwardly directed nozzle for spraying liquid droplets onto particles moving upward through said pipe.
From the top of the pipe the thus wetted particles reach the fluidized layer from the bottom of which the particles are again blown up through the pipe until the desired humidity has been achieved.
It is an essential feature of that process as well as of all other prior art fluidized bed granulation processes that although that part of the process in which the particles are wetted may be performed while a certain simultaneous drying takes place, the wetting takes place as a first step followed by a drying step. This means that the liquid sprayed onto the surface of the particles is present on/in the particles for a relatively long time, e.g. for more than 30 min, which leaves ample time for the liquid to penetrate into the interior of the particles. The liquid thus penetrated into the particles is obviously not at disposal on the particle surface for creating the stickiness essential for mutual adhesion between the particles which means that the total amount of liquid to be sprayed and evaporated increases.
This increased consumption of liquid not only impairs the heat-economy of the process but also causes operational problems.
Thus, especially when crystalline materials are granulated, the moisture content in the particles may suddenly exceed a value by which the particles are partly transformed into droplets causing a total caking of the whole batch being treated.
Also the high moisture content of the particles causes problems if a temporary interruption of the process is necessitated. If the flow of fluidizing gas is interrupted, the wet particles immediately collect as a mass on the bottom of the equipment. This mass can no longer be fluidized and must be removed by mechanical means or manually.
The fact that the wetting and the drying are performed to a substantial extent independent of each other involves the drawback that the result of the process cannot be evaluated until the total process is finalized. As explained above, the process cannot be stopped in a critical phase since the result thereof can be destructive and necessitate discharging of the whole batch.
Apart from these shortcomings of the fluidized bed granulation processes, the granulation results are not always satisfactory. often such processes deliver products having a too high proportion of non-agglomerated particles, even if the agglomeration process has been carried on to achieve a desired mean size of the granules.
It has now turned out that the above drawbacks related to the relevant prior art can be avoided or mitigated by a new process which inter alia involves that the particles to be granulated are subjected to collision promoting conditions and drying extremely short after having been wetted by spraying.
The process of the invention may be performed using equipment of a design similar to or even identical to equipment described for a completely different purpose, viz. coating of particles while avoiding any substantial granulation or agglomeration. Said equipment is described inter alia in U.S. Pat. No. 5,718,764 and termed PRECISION COATER(trademark) (Aeromatic-Fielder). Said U.S. patent is incorporated herein by reference. Details on this equipment will appear from the description below with reference to the drawings.
The invention deals with a process for granulation of a particulate material by subjecting the particles to a repeated circulating movement comprising an upward pneumatical movement from a starting area inside a vertical granulation pipe while the particles are entrained in a stream of drying and moving gas and simultaneously being subjected to a spray of droplets of granulation liquid, said circulating movement further comprising a downward movement outside said pipe, and a horizontal movement towards the starting area for said pneumatical movement, which process is characterized in passing the particles at said starting area into a zone above an annular horizontal aperture of a diameter less than the diameter of said vertical pipe, from which aperture a swirling flow of drying and carrying gas is emitted at a momentum accelerating and controlling the flow of particles in a rotation-symmet-rical, hollow zone above the annular aperture and around an imaginary vertical centre line of said aperture; and from a location on said imaginary centre line producing an upward spray of droplets of granulation liquid hitting the particles while these are in said zone thereby creating efficient contact between wet sticky particles combined with a fast drying to produce stable granules leaving the top of the granulation pipe in a non sticky condition.
It is essential features of this process that the active spray zone is small, only a few percent of the total amount of the batch is in the spray zone at any given time. When the particles pass into said zone above the annular aperture where drying and carrying gas is introduced, the product gets accelerated to ensure sufficient suspension so the individual particle can be wetted evenly. When the product leaves the exit at the top of the duct above the nozzle it has been wetted, particles have been sticked together to form granules, and these have been dried to a substantial extent.
By repeated passages through said pipe, further particles, viz. elementary particles and minor granules, stick to the already formed granules to increase their size as desired. Each granule may typically be subjected to from 20 to 1,000 passages through the pipe.
By the process of the invention it is possible to produce granulates which to the best knowledge of the inventor are not immediately obtainable by prior art granulation processes.
Therefore, the invention also deals with a granulate being the direct product of the process and characterized in having a content of non-granulated particles less than 5% by weight.
Fine lactose particles having a particle size below 38 xcexcm cannot be granulated satisfactorily by the prior art granulation methods discussed above, for which reason it is a further object of the invention to provide a lactose granulate having a mean granulate size above 200 xcexcm, consisting essentially of primary lactose particles having a particle size below 38 xcexcm.
Also fine particles of other crystalline materials are generally regarded as difficult to granulate, for which reason a further object of the invention is to provide a granulate of crystalline particles of high. solubility having a mean granulate size above 200 xcexcm and being essentially formed of crystalline primary particles of a mean particle size below 25 xcexcm.
The invention is elucidated further in the follow ing with reference to the drawings.